Animals are adorned with structural patterns that arise in highly specific places. In order to assure the consistent positioning of these structures, the developmental system must use dependable sets of regulatory factors specifying where the expression of particular genes must be activated or repressed. The developmental or genetic "address" conveyed by these regulators integrates various signals corresponding to segment identity, segment polarity, proximal-distal axis position, and sex. In the proposed experiments, the function of this developmental circuitry in specifying Drosophila melanogaster leg bristle patterns will be explored. Insect bristles function as peripheral sense organs and are a powerful model system for studying pattern formation in cellular fields. First, the expression patterns of major developmental address regulators will be characterized in the leg. Second, enhancer regions of the achaete-scute complex (AS-C) of proneural genes will be characterized as to their responses to developmental address regulators. Third, structures of enhancers needed for formation of specific leg bristle motifs will be investigated by identifying binding sites for known address-regulating proteins. Finally, the evolution of this developmental circuitry will be examined by cloning and characterizing AS-C enhancers from Drosophila species with divergent leg bristle patterns and examining their function in transgenic D. melanogaster lines. These studies will illuminate mechanisms by which position-specific mixtures of regulatory proteins are interpreted during pattern formation, as well as how these mechanisms have evolved to produce diversity among iterated appendages in a single species and among homologous appendages in different species.